This invention relates to a system for controlling the speed of an alternating current motor, more particularly, a system for controlling an AC motor capable of preventing current beats when the load frequency of the inverter approaches the frequency of an AC power source.
As will be described later in more detail, the conventional commutatorless AC motor is fed from a polyphase AC source of commercial frequency through a thyristor rectifier and a thyristor inverter which comprises a frequency converter. In the operation of such frequency converter, due to overlap angle and phase lag of ignition pulses supplied to respective thyristors, it is inevitable that the DC component contains a certain amount of AC components superposed thereon. Where the load frequency is close to the source frequency the ripple frequencies of the rectifier and inverter are close enough and the difference of these two frequencies is small. This causes a beat phenomenon of ripple voltage. Considering a current control system of such frequency converter, such phenomenon can be considered as a type of an external disturbance and causes a current beat and speed variation of the motor. Although various solutions have been proposed they are not efficient from the standpoint of performance and economy.